Phased-array antennas have been widely used as scanning antennas because they can be used to scan a wide area without changing the orientation of the antenna array. This is accomplished by incorporating a phase shifter in the feed line of each phased-array element. The resultant wave-front vector of the phased-array antenna is equal to the sum of the array element wave-front vectors. In order to produce a phased-array antenna with wideband frequency sensitivity, log-periodic arrangements of monopole elements have been successfully used as phased-array elements.
If properly designed, an antenna comprising a log-periodic arrangement of monopole elements is frequency independent over a given frequency band. Like other antennas of log-periodic design, the variation of performance is periodic with respect to the logarithm of frequency. Thus, the antenna is frequency independent to the degree that its performance variation with frequency is minimal over a log period. The frequency independence of the log-periodic design can be attributed to the geometrical progression of the monopole elements and their spacings. The heights, h, and the spacings, d, of consecutive monopole elements are related by a scaling factor, .tau.. EQU .tau.=h.sub.n-1 /h.sub.n =d.sub.n-1 /d.sub.n
A second design parameter, the spacing factor, .sigma., relates height to spacing for a cell within the log-periodic array. EQU .sigma.=d.sub.n /4h.sub.n
Theoretically, a log-periodic monopole antenna can operate over an infinite bandwidth. However, practical considerations require that such an infinitely long structure be truncated at both the high and low frequency ends, effectively establishing minimum and maximum operating frequencies. While operating at a frequency between the minimum and maximum operating frequencies, a log-periodic array can be divided into three functional regions: a transmission region, an active region, and an unexcited region. The transmission region consists of the monopole elements which are significantly less than a quarter wavelength in height; the unexcited region consists of the non-resonant monopole elements which are significantly greater than a quarter wavelength in height. The active region consists of the radiating monopole elements which are essentially a quarter wavelength in height. As the frequency is varied, the active region simply shifts to that portion of the array where the elements are approximately one-quarter wavelength in height.
In order to produce a directional wavefront, and to minimize excitation of higher-order resonances in the unexcited region of the array, the array is typically fed from the high-frequency end with a feedline element-to-element phase progression equal to or slightly less than a 180-degree, free-space phase shift.
A major objective of classical antenna design has been to reduce the physical size of the antenna without adversely affecting its sensitivity. For certain applications, the height of a phased-array antenna comprised of log-periodic monopole array elements is excessive. What is needed is an antenna design which incorporates all the advantages of this type of antenna, yet which is more compact.